I had to replace two sending units (both factory parts) in my 400E over a ten year period for the same symptoms you described above. It's not a difficult job at all. You will need a big crescent wrench to get to the nut on top of the gas tank from the trunk. Harbor Freight has a big wrench for cheap. All in all, it takes about 15 minutes.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles